From EngineHub.org Wiki
Since these instructions are all very version specific, make sure to use the same MC version for all the downloads and steps.
Using Mojang's game launcher
- Make sure to run the game at least once using the Minecraft version that you are going to use. Make a new profile with the desired MC version if you want.
- Install Minecraft Forge:
- Download the recommended Forge installer for the MC version.
- Run the installer and let it install Forge.
- Install LiteLoader: (optional, but recommended)
- Download the LiteLoader installer.
- Install LiteLoader, while making sure that you change the "Extend from" field so it selects a version with Forge.
- Restart the Minecraft launcher.
- In the bottom left of the launcher, choose the "Forge" profile.
- Click "Edit Profile"
- Click "Open Game Dir"
- Make a "mods" folder if one doesn't exist.
- Copy WorldEdit's .jar file into the mods folder, as well as WorldEdit CUI's .litemod file into the mods folder.
- Close any dialogs and click "Play" to launch the game with WorldEdit!
MultiMC is a free unofficial Minecraft launcher that makes mod installation easier.
- Download and run MultiMC.
- In the top left, click the new instance button in the toolbar.
- Enter a name and change the MC version as needed.
- Right click the newly created instance and click "Edit Mods"
- Click "Install Forge" and pick the latest version.
- Click "Install LiteLoader" and pick the latest version.
- On the left sidebar, click "Loader mods"
- Click "Add" and add both WorldEdit and WorldEdit CUI's files.
- Click Close and double click the instance to run the game. You will be asked to login to Minecraft.